In-School Sampling Frame

The clustered sampling design of Add Health is school-based for two reasons:

This is the best way to screen for respondents of interest.

With the school as a center, it is relatively easy to access the majority of respondents' peers, whose influences are fundamental to the study's hypotheses.

The primary sampling frame for Add Health is a database collected by Quality Education Data, Inc. Systematic sampling methods and implicit stratification ensure that the 80 high schools selected are representative of US schools with respect to region of country, urbanicity, size, type, and ethnicity. Eligible high schools included an 11th grade and enrolled more than 30 students. More than 70 percent of the originally sampled high schools participated. Each school that declined to participate was replaced by a school within the stratum.

Participating high schools helped to identify feeder schools—that is, schools that included a 7th grade and sent at least five graduates to that high school. From among the feeder schools, one was selected with probability proportional to the number of students it contributed to the high school. If the feeder school declined to participate, a replacement was selected. The recruitment effort resulted in a pair of schools in each of 80 communities (Some high schools spanned grades 7 through 12; for those, a separate feeder school was not recruited.) There are 132 schools in the core study.

In-School Questionnaire

The In-School Questionnaire, a self-administered instrument formatted for optical scanning, was administered to more than 90,000 students in grades 7 through 12 in a 45- to 60-minute class period between September 1994 and April 1995. There was no "make-up" day for absent students. Parents were informed in advance of the date of the questionnaire and could direct that their children not participate.

The questionnaire included topics such as these:

social and demographic characteristics of respondents (of interest both as data and as selection criteria for in-home special samples)

education and occupation of parents

household structure

risk behaviors

expectations for the future

self-esteem

health status

friendships

school-year extracurricular activities

Each participating school provided a student roster. Project staff assigned an identification number to each name and provided copies of the rosters to students for identifying their friends as they filled out the questionnaire. Rosters were collected at the end of the class period and destroyed.

In-Home Samples: Wave I

Main (core) sample

All students who completed the In-School Questionnaire plus those who did not complete a questionnaire but were listed on a school roster were eligible for selection into the core in-home sample. This is a nationally representative sample of adolescents in grades 7 through 12 in the US in the 1994–95 school year. Students in each school were stratified by grade and sex. About 17 students were randomly chosen from each stratum so that a total of approximately 200 adolescents were selected from each of the 80 pairs of schools. A total core sample of 12,105 adolescents was interviewed.

Special oversamples

Ethnic: Based on self-reported data from the In-School Questionnaire, four supplementary ethnic-group samples were drawn. Following are the numbers of completed cases in these samples:

1,038 blacks from well-educated families (with a parent with a college degree)

334 Chinese

450 Cuban

437 Puerto Rican

In addition, the main sample contains more than 1,500 Mexican-Americans and significant numbers of Nicaraguans, Japanese, South Koreans, Filipinos, and Vietnamese.

Saturation: To enable analysis of social networks, all enrolled students in 16 schools were selected for in-home interviews. These were two large schools (with a total combined enrollment exceeding 3,100) and 14 small schools (with enrollments of fewer than 300). One of the large schools is predominantly white and is located in a mid-sized town. The other is ethnically heterogeneous and is located in a major metropolitan area. The 14 small schools, some public and some private, are located in both rural and urban areas.

Disabled: A sample of 589 students self-reported on the In-School Questionnaire that they had physical disabilities involving the use of their limbs.

Adolescents were considered to be limb disabled if they indicated on the In-School Questionnaire (1) that they had difficulty using their hands, arms, legs, or feet because of a physical condition and (2) that they had used a mechanical device (e.g., wheelchair, cane, brace, or artificial limb) for the past 12 months or more. However, when the sampled adolescents were interviewed at home, many of them did not have limb disabilities. Therefore, it is questionable that those initially identified were actually disabled.

Genetic: The genetic sample consists of pairs of siblings living in the same households. Identical twins, fraternal twins, and half siblings were sampled with certainty. In addition, non-related pairs, such as step-siblings, foster children, and adopted (non-related) siblings, were part of the genetic sample. The majority of full-sibling pairs entered into the sample by chance (disproportionately drawn from the 16 saturation-school samples). The genetic sample will make possible analyses that differentiate between parental social influence and parental genetic influence, and analyses that assess the extent to which environmental influences on behavior are shared among siblings.

In-Home Interview: Wave I

In-home interviews were conducted between April and December 1995. All respondents received the same interview, which was one to two hours long depending on the respondent's age and experiences. The majority of interviews were conducted in respondents' homes. To protect confidentiality, no paper questionnaires were used. Instead, all data were recorded on laptop computers. For less sensitive topics, the interviewer read the questions aloud and entered the respondent's answers. For more sensitive topics, the respondent listened through earphones to pre-recorded questions and entered the answers directly. In addition to maintaining data security, this minimized the potential for interviewer or parental influence.

The following topics are covered by the In-Home Interview:

health status

health-facility utilization

nutrition

peer networks

decision-making processes

family composition and dynamics

educational aspirations and expectations

employment experience

the ordering of events in the formation of romantic partnerships

sexual partnerships

substance use

criminal activities

Care was taken to screen respondents on age and experience so that only appropriate questions were asked. Additional questions concerning the co-occurrence of risk behaviors were asked of respondents who indicated multiple behaviors, for example, fighting while using drugs or drinking while carrying a weapon.

Vocabulary Data

At the beginning of the interview, respondents were given the Add Health Picture Vocabulary Test (AHPVT), a computerized, abridged version of the Peabody Picture Vocabulary Test—Revised. In this test, the interviewer reads a word aloud and the respondent selects the illustration that best fits its meaning. Each word has four simple, black-and-white illustrations arranged in a multiple-choice format. For example, the word "furry" has illustrations of a parrot, dolphin, frog, and cat from which to choose. There are 87 items on the AHPVT, and raw scores have been standardized by age.

Spatial Data

Interviewers used hand-held Global Positioning System (GPS) devices to collect latitude and longitude readings for each household lacking a geocodable street address (approximately 25 percent of Wave I households). This enabled complete geocoding for almost all (98 percent of) Wave I addresses. The location information was transformed into a geographic distribution of adolescent respondents around a central point in the community. Additionally, the geocodes provide necessary links to the contextual information.